Gas tungsten arc welding (GTAW), also called tungsten inert gas (TIG) welding, is a well known method for welding metals, in particular aluminum and magnesium. A description of GTAW, including the problems associated with welding aluminum and magnesium, may be found in Welding Handbook, Chapter 3, Gas Tungsten Arc Welding, pages 74-107, 1991, published by the American Welding Society.
It is well known in GTAW to use an ac squarewave power source, which is particularly suited for welding aluminum and magnesium. An ac squarewave power source provides a single phase output having a straight polarity (electrode negative) half-cycle and reverse polarity (electrode positive) half-cycle, with a rapid transition between half cycles.
In GTAW welding a squarewave having independently adjustable duration of the two half-cycles (positive and negative) is desirable. In other words, it is desirable to have the positive half cycle duration be controllable to be longer or shorter in time than the negative half-cycle. Hereinafter independent control of the positive and negative half-cycle widths is called adjustable balance. It is also desirable to independently control the amplitude of the two half-cycles, i.e. control the positive half-cycle amplitude to be greater or less than the negative half-cycle amplitude (hereinafter independent amplitude control).
One ac squarewave power source is described in U.S. Pat. No. 4,038,515, (the '515 patent) issued Jul. 26, 1977 to Robert L. Risberg. The device disclosed in the '515 patent uses a single phase input and provides an ac squarewave welding current output, with adjustable balance, i.e. the duration of the electrode positive and electrode negative half-cycles may be adjusted with respect to one another.
However, a power supply that uses a single phase input such as the power supply of the '515 patent requires a very large single phase input current requirement. Such a large single phase current draw is undesirable for industrial sites having three-phased power because it unbalances the power lines.
There are prior art GTAW power supplies using an inverter with a three phase input that have the capability of independent control of the output half-cycles. However, many prior art GTAW inverter power supplies directly rectify the input line. Thus, to design inverters for multiple input voltages operating directly from the power lines requires compromises in the design, such as dual inverters with a series parallel connection, or over current and over voltage ratings on the power components to be re-linkable for the various inputs. Moreover, such designs provide an input power factor that is highly capacitive. Such an input power factor can cause neutral unbalance and line notching. Accordingly, it is desirable to provide an inverter for GTAW that provides a constant secondary voltage, regardless of the input voltage and that provides a relatively high power factor for a welding power source.
There are additional problems with known inverter GTAW power supplies. Inverter power supplies for GTAW generally provide a substantially triangular wave shape load current. Due to this triangular wave shape, in a welding application, or any other application where wide range of output currents is required, it can be costly to size the output inductor to both the minimum and maximum current required by the load, unless current discontinuities can be accommodated.
For example, a welding power supply may be required to go from 20 amps or less to 300 amps or more. In order to accommodate a low current, an inductor must be very large. It is costly to provide an inductor that will accommodate this wide range of currents, without causing discontinuities in the output current. Moreover, a small inductor is desirable for applications which require pulsing and output because a large inductance will cause the slewing rate of the output to be slow, adversely affecting the shape of pulsed or squarewave outputs.
Unfortunately, the discontinuities in the output current will appear as an arc outage which is generally not acceptable in GTAW. Accordingly, it is desirable to provide an inverter power supply for GTAW that is capable of providing a wide range of output currents, yet does not provide output current discontinuities and still does not require the use of a costly large inductor.
When providing a GTAW inverter power supply it is desirable to provide the user with the option of selecting straight polarity, reverse polarity or alternating polarity welding. Such a machine has a wider range of applications and is more desirable to the purchaser. It is well known in the prior art that it is difficult to start an arc when the electrode is negative. Conversely, when the electrode is positive the arc will start much easier. However, most dc GTAW is done with the electrode negative, when it is difficult to start.
Accordingly, it is desirable to provide a GTAW inverter power supply that provides an arc starting capability wherein the arc is started with a brief electrode positive polarity followed by a rapid reversal to electrode negative polarity for the welding process. Additionally, when using ac GTAW for the reasons stated above it is desirable to provide an extended electrode positive period in order to facilitate arc starting.
It is also well known in the art that in ac GTAW the arc extinguishes with every current reversal. When the reversal is in the direction of electrode emitting (from electrode positive to electrode negative), the arc will reignite without much difficulty because of the thermionic state of the electrode. However, when the reversal is to the workpiece emitting (from electrode negative to electrode positive), arc reignition problems are encountered. The workpiece is generally below thermionic temperature. Also, the molten weld pool is very clean metal due to the ionic bombardment during past electrode positive half cycles. Both of these factors contribute to difficulty in reestablishing the electrode positive half cycle.
One way the prior art has compensated for this difficulty is by providing high frequency overlays. However, there are many problems associated with such high frequency overlays, one being that high frequency can provide undesirable noise which adversely affects the operation of control circuits, telecommunication equipment and computer equipment. Accordingly, it is desirable to provide a GTAW power supply that will provide for arc starting (stabilizing), when trying to reestablish the electrode positive half cycle without the use of superimposed high frequency.
An additional problem with inverter power supplies is that a considerable amount of acoustic noise is generated by current reversal. Moreover, this acoustic noise is greater for greater welding currents. Accordingly, it is desirable to provide a GTAW power supply that will provide for a lower output current immediately before current reversal.
Inverter power supplies will have a distributed (parasitic) inductance in the load (from the weld cables and arc e.g.). This parasitic inductance will slow the transition at current reversals, adversely affecting machine performance. A high forcing voltage will cause a rapid current reversal. However, a high forcing voltage may cause damage to switches in the inverter. Thus it is desirable to provide a forcing voltage sufficiently high to cause a smooth and fast current reversal, but not so high as to damage switches.
Finally, it is well known that there is an additional safety factor with respect to ventricular fibrillation, the major cause of electrocution, of dc when compared to ac. This factor is given in IEC 479 as 3.75. Accordingly, it is desirable to provide a GTAW power supply that provides a dc voltage at the terminals when not welding or when the power source is at open circuit.